If build directory contains nothing specific, you can issue rm -r * in it each time on build (btw, ensure that you’re in right place invoking it!).
That is the simplest step, it works always.
In general it is enough to delete CMakeCache.txt in build folder (and it is also cure if you once made in-source build, and so it is no more possibly to make build in another place because of this file created in sources).